An acrylic pressure-sensitive adhesive has been widely used as a pressure-sensitive adhesive having excellent pressure-sensitive adhesive force, cohesive force, and aging resistance. In particular, recently, the demand of the pressure-sensitive adhesive tape for heat resistant uses such as a fixing tape for bonding parts or a masking tape in a soldering step at assembling electronic parts has been increased.
An acrylic pressure-sensitive adhesive is generally produced by copolymerizing an acrylic acid alkyl ester having 2 to 14 carbon atoms in the alkyl moiety as the main component and, if necessary, a small amount of a polar component such as acrylic acid, etc., and a monomer having a high glass transition temperature, such as styrene, vinyl acetate, etc., to balance peeling characteristics, tack, etc. Also, the copolymer is crosslinked for the purposes of improving heat resistance and cohesive force.
In producing such an acrylic pressure-sensitive adhesive, usually, after solution-polymerizing a monomer in an organic solvent, a tackifying resin, a crosslinking agent, etc., are compounded with the polymer solution formed to obtain an adhesive composition, and after coating the adhesive composition on a substrate, a large amount of the organic solvent is removed by heating to obtain an adhesive sheet or tape.
However, in the conventional production, the average molecular weight of the polymer or copolymer obtained is at most 1,000,000 due to the chain transfer into the organic solvent at the polymerization. Therefore, if the polymer or copolymer is not crosslinked, the adhesive lacks in the retention performance, and even if the polymer is crosslinked, when the temperature becomes higher than 100.degree. C., the cohesive force is decreased and the retention performance is greatly decreased.
Accordingly, when the acrylic pressure-sensitive adhesive is used for a fixing tape or a masking tape in a soldering step, there is a problem that slipping of the tape and oozing of the adhesive occur during use. Furthermore, there are problems that a low-boiling organic solvent used for the production of the adhesive remains in these tapes, which evaporates and expands at a high temperature to cause foaming, swelling, slipping, etc., at the adhered surface, and also electronic parts are corroded by the evaporated gas.
On the other hand, recently, from a standpoint of a safety and an environmental hygiene caused by the use of an organic solvent, photopolymerized type acrylic pressure-sensitive adhesives have been proposed. For example, U.S. Pat. No. 4,181,752 discloses a method for preparing a pressure-sensitive adhesive without using an organic solvent by photopolymerizing an acrylic acid alkyl ester and a modifying monomer on a substrate. Also, a transfer-type pressure-sensitive adhesive obtained by forming the pressure-sensitive adhesive similar to the above pressure-sensitive adhesive on a releasing liner and transferring the adhesive onto a substrate is known.
By such a photopolymerized type pressure-sensitive adhesive, the above-described various problems caused by the use of an organic solvent are all avoided and also the molecular weight of the polymer can be increased by irradiating light having a relatively weak intensity. Therefore, it has been expected that a pressure-sensitive adhesive having a high degree of crosslinking and a large cohesive force is obtained.
However, since the above-described conventional photopolymerized type pressure-sensitive adhesive is photopolymerized in a complete bulk state, the rate of polymerization by no means becomes 100% by weight and several percent by weight of unreacted monomer remains. The unreacted monomer not only becomes the cause of an unpleasant odor but also functions as a plasticizer of the adhesive to cause great decrease in the cohesive force at high temperature and cause swelling of the adhered surface and the contamination of a gas by evaporation of the monomer at high temperature. This has refused use of the adhesive in heat-resistant uses.
For decreasing the amount of the unreacted monomer, it may be considered to improve the rate of polymerization by using a large amount of a photopolymerization initiator, but since in such a case, with increase of a radical concentration of a photopolymerization initiator, the molecular weight of the polymer formed is decreased, and the cohesive force of the pressure-sensitive adhesive at high temperature is decreased. Hence a high heat resistance desired for a soldering step, etc., is not attained.
JP-A-2-60981 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses that by conducting the photopolymerization for producing a pressure-sensitive adhesive tape in two stages, the amount of the residual monomer is decreased and the productivity is improved by improving the rate of polymerization.
However, in this method, there are disadvantages that since the residual monomer is consumed in the 2nd stage, it is required to irradiate with light having a intensity higher than that of the light in the 1st stage, whereby the formation of low molecular weight materials cannot be prevented and hence although the pressure-sensitive adhesive has a heat resistance of about 100.degree. C., in a heat resistant soldering use at higher temperature, bleeding of the low molecular weight materials causes prints and stains at the back surface of the pressure-sensitive adhesive tape and also causes peeling off, slipping, and falling of the pressure-sensitive adhesive tape.
Furthermore, recently, the requirement for a heat resistant use tends to more increase and in particular, the durability and reliability of a pressure-sensitive adhesive used under high temperature for a long period of time have been required. For example, in the case of using the adhesive under high temperature for a long period of time, the decomposition and oxidation of the pressure-sensitive adhesive occur and by hardening and flowing out of the pressure-sensitive adhesive and the generation of decomposed gases, decrease in the performance of the pressure-sensitive adhesive and contamination of environmental portions become a problem. The conventional pressure-sensitive adhesive has a high degree of crosslinking and a large cohesive force as described above, but is insufficient in the point of stability when it is used at high temperature for a long period of time, and from this standpoint, use of a conventional pressure-sensitive adhesive in heat resistant uses has been obstructed.